Electrical feedthroughs are used to connect an electrical power source to downhole logging tools and the like which determine physical, chemical, and structural properties of the formation. However, a downhole environment is subjected to a variety of harsh liquid environments such as brine, gas, and oil fluid that often contains water and moisture. An electrical feedthrough typically comprises metal pin(s), sealed in an insulating material which may carry substantial amounts of power with signals up to a few thousand volts that requires a high insulation resistance. It is clear that if the moisture is permitted to pass into the downhole logging tool enclosure due to failed hermetic seal of the feedthrough, it may lead to catastrophic electrical breakdown.
Downhole logging tool and electrical circuits are packaged in a hermetically-sealed metal enclosure, which is either pressurized, or filled with fluid, to protect the circuits from downhole corrosive environment and humidity. The sealed tool enclosure uses an electrical feedthrough that transmits the power to inside electronics or sends the measured downhole data to surface instruments. For permanent installations in the downhole environment, it is important that these electrical feedthroughs are reliable. In particular, it is important that the downhole fluid is prevented downhole fluid and moisture from penetrating the electrical feedthroughs because the presence of the conductive fluid, such as seawater or brine in the electrical feedthroughs can cause a short circuit in the system. In one case, a sealing material may be of high dielectric strength but lack of mechanical strength and appropriate coefficient of thermal expansion that may lead to sealing material cracks by high stress. In another case, a sealing material may be of high mechanical strength and a matched coefficient of thermal expansion to metal enclosure but lack of sufficient electrical insulation to survive downhole temperature and pressure without failures. In also further case, a sealing material may have high dielectric and mechanical strength and also matched coefficient of thermal expansion to metal enclosure but lack of moisture resistance that could also lead to electrical breakdown by moisture deteriorated electrical resistivity.
Aromatic polyether ketones (PEEK, PEK, PAEK, and PEKK) based organic polymers are first type of dielectric sealing materials that widely used in an electrical feedthrough seal for subsea and downhole logging tools. Typically, in low temperature installations, aromatic polyether ketones based polymer materials are used as the pressure barrier and insulating component. However, the structural integrity as well as the dielectric strength of aromatic polyether ketones can be compromised at higher temperatures because of low glass transition temperature of Tg<150° C. Under long-term exposure to high pressure and temperature and corrosive fluids and frequent thermal cycles during the deployment from downhole to surface, the hermetic seals will eventually fail, at least allowing fluid and moisture to enter the pressure bulkhead and reach the contact pins. If the invading fluid is conductive, which is usually the case in downhole and subsea environments, a short circuit may occur in the logging tool system, resulting in power and data loss. One the other hand, although these thermoplastic materials have high dielectric strength, their ambient water absorption of ˜0.5% could slowly degraded under moisture-rich downhole or subsea environment, even without moisture passing through the conducting pin surface.
Inorganic glasses and glass-ceramics (Corning 7070, 58061, EG2927, Li2O—Al2O3—SiO2, MgO—Al2O3—SiO2, and ZnO—Al2O3—SiO2 etc.) are second type of dielectric sealing materials that have high dielectric strength, electric resistivity, mechanical strength, and break-down voltage. Despite a great success in many glass-to-metal seal systems, these glass and glass-ceramic sealed electrical feedthroughs often failed not due to mechanical stress but due to the deterioration of the electric insulation. One failure mode is that the sealing material is of a hydrophilic nature due to its porosity that leads to absorption of moisture or water and eventual short circuit. There is still a need for developing a high moisture resistant dielectric sealing material sealed electrical feedthrough that enables reliable operation under 30,000 PSI/200° C. hostile water-based or moisture-rich downhole and subsea environments.
It is desirable for having a high mechanical strength and high dielectric strength with moisture-resistant dielectric sealing material for downhole electrical feedthrough package that not only provides high glass-to-metal seal strength against potential mechanical failures but also provides high electrical insulation against potential electrical failures even at moisture-rich downhole environment. The present invention relates to the high strength dielectric sealing material sealed electrical feedthrough package in general, and to the moisture-resistant dielectric sealing material sealed downhole electrical feedthrough in particular for enabling downhole logging tools, LWD and MWD tools reliable operation in water-based or moisture-rich oil-based wellbores.